Concrete and its reinforcement

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3.1.6Concrete specification

Concrete shall be specified correctly to ensure adequate strength and durability. Issues to be taken into account include:

  1. concrete in non-hazardous conditions
  2. exposure to climatic and atmospheric conditions
  3. exposure to aggressive ground conditions
  4. exposure to sulfates and acids in groundwater
  5. effects of chlorides
  6. effects of alkali-silica reaction
  7. aggregates.

Concrete mixes should be suitable for particular end uses and specified in accordance with BS 8500-1 as either:

  • designated mix, which is supplied ready mixed, or
  • standardised prescribed mix for site mixing.

Designated mixes should conform to Table 6 of BS 8500-2 2015. Standardised prescribed mixes should conform to Tables 2 and 3 in this chapter.

Mixes should also be designed for the expected conditions of the geographical location of the site and the location of the concrete element in the structure. Higher grade concrete has greater resistance to chemical and mechanical damage and should be specified accordingly.

In addition to the issues in this section, durability is reliant on:

  • correct control of the water:cement ratio
  • full compaction of the placed concrete
  • good curing.

Concrete in non-hazardous conditions

Table 4: Minimum specifications for general purpose concrete mixes

Location and useBS 8500 and BS EN 206
Ready-mixed concrete (designated mix)Site-mixed concrete (standardised prescribed mix)Consistence class
Substructure and ground floors
■ Rough blinding (non-structural).
■ Infill.
■ Unreinforced oversite concrete below timber floors.
GEN1ST2S3
■ Structural blinding and over break.
■ Strip foundations.
■ Trench fill.
■ Other mass concrete foundations.
■ Fill to wall cavity.
■ Solid filling under steps.
GEN1ST2S3/S4(1)
■ House floors not designed as suspended and not reinforced:
- Permanent finish to be added, e.g. screed or floating floor.
- No permanent finish to be added, e.g. carpet.

GEN1
GEN2

ST2
ST3

S2
S2
■ Garage floors not designed as suspended and not reinforced.GEN3ST4S2
■ House and garage ground floor slabs:
- Fully or nominally reinforced, either ground bearing, suspended or over sub-floor voids.
RC28/35ST5(2)S2
Superstructure
■ General reinforced concrete exposure class(3) to BS8500-1:
- Nominal cover to reinforcement of 35mm (which is the minimum cover of 25mm plus an allowance in design for deviation of 10mm).
RC25/30(4)S2
- XC1 (dry) and XC2 (wet, rarely dry).

- XC3 (moderate humidity), XC4 (cyclic wet and dry) and XF1 (freeze/thaw attack and no de-icing agent).
RC32/40S2
- Nominal cover to reinforcement of 40mm (which is the minimum cover of 30mm plus an allowance in design for deviation of 10mm).

- Any exposure class (XC1-4 and XF1).
RC28/35(5)S2
In-situ external concrete
■ Drives and paths.
■ Foundations for precast concrete paving slabs.

PAV1
GEN1

ST5(6)
ST1

S2
S1

Notes

1     Consistence class S3 should be used for strip foundation concrete and consistence class S4 should be used for trench fill foundation concrete.
2     ST4 mix for house and garage floors may only be used in conjunction with Chapter 5.2 ‘Suspended ground floors’. In all other cases, the designated mix should be used.
3     Exposure classes (XC1-4 and XF1) are defined in BS 8500-1 Table A.1.
4     In this situation, ST4 mix may be used only for small quantities of concrete. In all other cases, the appropriate designated mix should be used.
5     In this situation, an ST5 mix may be used only for small quantities of concrete. In all other cases, the appropriate designated mix should be used.
6     Not suitable in areas of severe exposure to frost attack. This is equivalent to exposure class XC4 above.

Exposure to climatic and atmospheric conditions

Any concrete mix should be designed for the conditions expected at the geographical location of the site and at the location of the element in the structure.

Table 5: Exposure classes and examples of where they may occur, based on Table 1 of BS EN 206

Exposure classEnvironmentExposure conditions
XC1Dry or permanently wetConcrete inside buildings with low air humidity.
Concrete permanently submerged in water.
XC2Wet, rarely dryConcrete surfaces subject to long-term water contact. Many foundations.
XC3Moderate humidityConcrete inside buildings with moderate or high air humidity.
External concrete sheltered from rain.
XC4Cyclic wet and dryConcrete surfaces subject to water contact, not within exposure class XC2.
XF1Moderate water saturation, without de-icing agentVertical concrete surfaces exposed to rain and freezing.

Concrete in aggressive ground

Mixes should conform to BS 8500. The information in this section describes minimum specifications for lower range ‘chemical aggressiveness’. Specialist advice should be sought for more aggressive conditions.

Table 6: Aggressive chemical environment for concrete (ACEC) site classification(1) and applies to concrete exposed to ground with a pH value greater than 2.5

Notes

1     For concrete quality and APM for ACEC classes above AC-2z, follow specialist advice. For the full list of ACEC classes, refer to Table A.2 of BS 8500-1 or BRE Special Digest 1 Table C1 for natural ground locations, and Table C2 for brownfield locations.
2     Applies only to sites where concrete will be exposed to sulfate ions ( SO₄ ), which may result from the oxidation of sulfides such as pyrite, following ground disturbance.
3     Applies to locations on sites that comprise either undisturbed ground that is in its natural state or clean fill derived from such ground.
4     ‘Brownfield’ is defined as sites which may contain chemical residues remaining from previous industrial use or from imported wastes.
5     An additional account is taken of hydrochloric and nitric acids by adjustment to sulfate content.
6     For flowing water that is potentially aggressive to concrete owing to high purity or an aggressive carbon dioxide level greater than 15mg/l, increase the ACEC class to AC-2z.

Explanation of suffix symbols to ACEC class number:

  • Suffix ‘s’ indicates that, as the water has been classified as static, no additional protective measures are generally necessary.
  • Concrete placed in ACEC classes which include the suffix ‘z’ have primarily to resist acid conditions and may be made with any of the cements or combinations listed in Table D2 of BRE Special Digest 1.

This table is based on Tables C1 and C2 of BRE Special Digest 1.
The information in Table 7 provides guidance on selecting mixes for concrete elements in aggressive ground.

Table 7: Design guide for concrete elements in the ground

Concrete elementACEC class(1)Designated mix
Strip or trench fill foundation, raft foundation, pile(3) and ground beams.AC-1, AC-1sAs Table 4
AC-2, AC-2sFND2(2)
AC-2zFND2z(2)

Notes

1 For all other ACEC classes, follow specialist advice.

Exposure to sulfates and acids in groundwater

Sulfates, chemicals and high acidity can cause expansion, cracking and damage to concrete. Where ground water is highly mobile, or where concrete is at risk from chemical attack, the level of sulfate and other chemicals should be determined according to the ACEC class (aggressive chemical environment for concrete class) and BRE Special Digest 1.
For higher ACEC classes, specialist advice should determine the design chemical class (DC class) and appropriate additional protective measures (APM) where required. Table A.7 of BS 8500-1 should be used to select the mix specification.
For lower ACEC classes (AC-1,AC-1s, AC-2, AC-2s and AC-2z), information in Tables 6 and 7 should be used to select the mix specification.

Effects of chlorides

Chlorides, which are contained in all concrete materials, increase the risk of corrosion in metal and can reduce the chemical resistance of concrete, therefore chloride content of fresh concrete should be limited in accordance with BS EN 206 Table 15.
Cured concrete can be damaged by chlorides in the ground, sea spray, or products used for de-icing highways, and specialist guidance should be followed.

Effects of alkali-silica reaction

Alkalis can cause expansion, cracking and damage to concrete. Damage can occur when all the following conditions are present:

  • a source of alkali
  • a high moisture content
  • where the aggregate is alkali reactive.

Alkali content calculated in accordance with BRE Digest 330 or Concrete Society Technical Report 30 should not exceed 3kg/m³. Where unfamiliar aggregate materials are used, special precautions may be required.
Standardised prescribed mixes should conform to BS 8500.

Aggregates

Aggregates should be of a grade which ensures adequate durability of the concrete. Certain types of aggregate are shrinkable and require special precautions in mixing. Certain types of aggregate may be susceptible to alkali attack or excessive moisture movement.
Proprietary and recovered aggregates should only be specified where they have been assessed in accordance with Technical Requirement R3.